PROJECT SUMMARY/ABSTRACT Medullary Thyroid Cancer (MTC) is a progressive, incurable disease with a 40% 10-year survival rate for metastatic cases. Hereditary MTC is caused by oncogenic mutations in the RET gene. Treatment options for progressive MTC are very limited and resistance develops to therapies such as Vandetanib. Thus, it is crucial to explore new therapies for Ret mutant and non-mutant cases, including those who develop resistance to current treatments. Sporadic forms can be caused by somatic mutations in RET, but 60% of sporadic MTCs do not harbor RET mutations. Mouse models show that MTC can arise after loss of negative regulators of the CDK/RB pathway such as Rb, p16, p18, and p27. IHC of human MTC samples reveal increased nuclear levels of RB, and array CGH data show loss of heterozygosity at the p18 and E2F2 loci, including those with and without RET mutations. These findings indicate that CDK/RB pathway activation is functionally relevant in human MTC. Our lab has used Dinaciclib, an inhibitor of CDK 1, 2, 5, and 9, to target the CDK/RB pathway in MTC cell lines (TT and MZ-CRC-1) expressing Ret mutants (C634W and M918T) seen with clinically. Results showed a remarkable reduction in cell viability and proliferation in a dose and time-dependent manner at low concentrations (IC50: 5nM for TT and 30nM for MZ). The TT cell line, which is more sensitive to the treatment, lacks p18 expression. Treatment of Vandetanib-resistant MTC lines also resulted in decreased viability. Activation of apoptosis was evidenced by PARP cleavage. Expression of CDK 1, 2, and 5 was stable; however, CDK9 mRNA and protein expression remarkably decreased at doses where cell viability was compromised and apoptosis activated. In addition, Ret levels were also reduced in concert with loss of CDK9, suggesting they may be linked. We then evaluated CDK9 in human samples and discovered a high frequency of CDK9 gene amplification (34%) and increased IHC CDK9 staining (79%) in MTC, including those with RET gene mutations. Predicted regulators of CDK9 gene transcription such as Stat3 and NFkB subunits are reported to be upregulated in human MTC and are activated by Ret signaling. We therefore hypothesize that CDK9 is mechanistically important in MTC and in its response to Dinaciclib and that p18 levels predict response to CDK inhibitors in MTC. Our aims are 1) To determine if Ret-mediated enhanced CDK9 expression is the mechanism for MTC cell sensitivity to Dinaciclib and determine if increased CDK9 levels or gene amplification predict aggressive MTC tumor behavior, 2) to determine the effect of Dinaciclib in Vandetanib-resistant progressive MTC in-vivo, and 3) to determine the role of p18 loss in cell sensitivity to CDK inhibitor treatment. The long term goal of this research is to reveal new therapeutic targets and predictors of response for progressive MTC, including Vandetanib-resistant tumors.